1. Field of the Invention
The present invention relates to a semiconductor light-emitting device having a plurality of light-emitting regions formed by diffusion of an impurity of a second conductive type into a substrate of a first conductive type, and more particularly to a structure and fabrication method that enable the light-emitting regions to be arranged more densely than before.
2. Description of the Related Art
Known semiconductor light-emitting devices include arrays of light-emitting elements such as arrays of light-emitting diodes, generally referred to as LED arrays. LED arrays formed on semiconductor chips are used as light sources in, for example, electrophotographic printers.
FIGS. 28A and 28B show an LED array disclosed on page 60 of the book LED Purinta no Sekkei (Design of LED Printers), published by Torikeppusu. FIG. 28A shows the cross-sectional structure of the LED array; FIG. 28B is a plan view showing the chip pattern.
The LED array shown in these drawings has a plurality of light-emitting regions 3. The light-emitting regions 3 are formed by growing an epitaxial layer 2 of a first conductive type (an n-type GaAs0.6P0.4 layer) on a gallium-arsenide (GaAs) substrate 1 of the first conductive type (n-type), then selectively diffusing an impurity of a second conductive type (p-type), such as zinc (Zn), into the epitaxial layer 2. Each light-emitting region 3 has an individual aluminum (Al) electrode 4, and the light-emitting regions 3 share a common gold-germanium-nickel (Au—Ge—Ni) electrode 5. The individual electrodes 4 are formed on an insulating layer 6 deposited on the epitaxial layer 2, and make electrical contact with the surfaces of the light-emitting regions 3. The common electrode 5 is formed on the underside of the n-type GaAs substrate 1.
There is an increasing demand for electrophotographic printers capable of printing very clear images. Improved clarity is obtained by increasing the resolution of the printer. For an LED printer, this means increasing the resolution of the LED arrays used as light sources, by increasing the density of the layout of their light-emitting elements.
FIG. 29 illustrates relationships between the size and density of the light-emitting elements in an LED array. FIG. 30 shows the relationship between the width of the diffusion areas and the diffusion depth, indicating width in arbitrary units on the horizontal axis, and depth in arbitrary units on the vertical axis.
In the arrays shown on the left in FIG. 29, the width (in the array direction) of a light-emitting region 3 is equal to the distance between two adjacent light-emitting regions 3. The resolution of the array is equal to twice this value. Accordingly, to increase the resolution, it is necessary to decrease the size of the light-emitting regions 3, which entails decreasing the size of the diffusion windows through which the light-emitting regions 3 are formed. As shown in FIG. 30, however, if the size of the light-emitting regions 3 is decreased beyond a certain point, the diffusion depth must also decrease. Because of inadequate diffusion depth, the light-emitting regions 3 then fail to emit the desired amount of light.
Referring once more to FIG. 29, it can be seen that as the size of the light-emitting regions 3, 3a, 3b is decreased to increase the density of the array, eventually the light-emitting regions become too small to be formed with adequate depth. If the density is increased without changing the size of the light-emitting regions 3, however, it soon becomes impossible to fabricate the array because adjacent light-emitting regions overlap.